Set AC_CONFIG_AUX_DIR to build directory

Set AC_CONFIG_AUX_DIR to build directory

The Autoconf macro AC_CONFIG_AUX_DIR can set the location of the
auxiliary build tools such as config.guess, config.sub, and bundled
libtool scripts and moves these bundled files from the root directory
to the build subdirectory.

Additionally some changes in this context or as a part of obsoletion:
- The LT_TARGETS variable in build/build2.mk file was once used as a part
  of the Automake step. It's not used anymore and has been refactored to
  separate makedist script directly.
- ltconfig is not used anymore since libtool 1.4+
  cf8d1563c27a70fdd24055c1e80218ec7b5962d6
- phpize file locations for the config.guess, config.sub, and ltmain.sh
  has been refactored accordingly.